The present invention relates to a jitter analyzer for analyzing what are called jitters of input pulses, such as the pulse width fluctuation and pulse repetition period fluctuation.
It is coded pulse train signals that are reproduced, for example, in a magnetic disk unit for use as an external storage of a personal computer, a digital audio apparatus called a compact disk apparatus, or a video apparatus such as a laser disk apparatus. If a jitter is contained in such a coded pulse train signal, an error is likely to occur during decoding of the signal. The occurrence of an error is not allowed in the case of an external storage of a computer, in particular. In the case of determining if these digital apparatuses are good or defective, it is therefore necessary to measure whether the jitter of the pulse train signal stays within a given limit or not.
In Japanese Patent Application Public Disclosure No. 284268/87 the inventor of this application proposes a pulse width fluctuation analyzer. FIG. 1 shows its basic construction in a simplified form. Input pulses are applied to a time window 2, whereby pulses of a particular pulse width are sequentially extracted or selected from among pulses of various pulse widths which are substantially integral multiples of a unit pulse length, and the selected pulses are converted by a time-to-voltage (hereinafter referred to as T-V) converter 3 to voltages corresponding to the pulse widths. The voltage thus converted for each pulse is provided to an ordinary fast Fourier transform (FFT) analyzer 4, wherein it is subjected to a fast Fourier transform for analysis. In this way, pulse width fluctuations or jitter components of the input pulses can be detected, and in particular, even if the jitters vary with a plurality of periods, components of the respective periods can be detected, which can be used to ascertain the cause for the occurrence of each jitter.
With the T-V converter 12 which simply integrates reference clock pulses for the duration of the input pulse as in the above-mentioned Japanese patent public disclosure, it is impossible to achieve a high precision time-to-voltage conversion, for example, with a resolution of 100 ps. In practice, as proposed by the inventor of this application in Japanese Patent Application Public Disclosure No. 294993/87, for instance, a constant voltage is integrated for a fractional or odd period .DELTA.T.sub.1 from the rise of a certain input pulse to the rise of a clock pulse immediately thereafter and for a fractional or odd period .DELTA.T.sub.2 from the fall of the input pulse to the rise of a clock pulse immediately thereafter to thereby obtain voltages corresponding to odd or fractional components of the clock pulses at the rise and fall of the input pulse, respectively, the voltage at the fall of the input pulse is subtracted from the voltage at the rise thereof to obtain a difference voltage, and the number of rises of the clock pulses, counted from the end of the fractional period .DELTA.T.sub.1 to the end of the fractional period .DELTA.T.sub.2, is converted by a D-A converter to the corresponding voltage. The sum voltage, obtained by adding the difference voltage and the voltage corresponding to the clock count value, is sampled and held, as a voltage corresponding to the width of the input pulse, by a sample-hold circuit and then output therefrom. In this instance, the number of digits for conversion is a predetermined sequence of four digits or so, and in the case of voltage conversion of, for example, a 1.2345 .mu.s pulse width, a voltage corresponding to the value of the low order four digits 2345, the least significant digit of which is 100 ps, is provided, that is, the voltage output is yielded with no information of the most significant digit being taken into account. This does not matter when it is necessary to analyze the pulse width fluctuation, i.e. the frequency component of the jitter and the pulse width of the input pulse is preknown.
Since the magnitude of the jitter relative to the pulse width is often required, it is necessary to measure the pulse width of the input pulse when it is unknown. Moreover, in the case where input pulses of different pulse widths are provided and it is desirable to detect the frequency component of a jitter corresponding to each pulse width or the distribution of frequency of the jitter with respect to the pulse width, it is necessary to measure the pulse width.
To this end, the conventional jitter analyzer shown in FIG. 1 is provided with a pulse width measuring part 5, wherein the width of the output pulse from the time window 2 is measured. The pulse width measuring part 5 can be operated in synchronism with the sample-hold timing of the T-V converter 3, since the timing period with which pulses of a fixed width are extracted is not always constant when the time window 2 is used, the measuring cycle is not constant, in general, that is, the measurement dead time varies. This means that the sample-hold timing in the T-V converter 3 does not become constant, and therefore it is impossible to use, as a sampling clock for fast Fourier transformation, a signal synchronized with the sample-hold in the T-V converter 2. Accordingly, the FFT analyzer 4 has an independent sampling clock of a fixed interval. In other words, the T-V converter 3 and the pulse width measuring part 5 can be operated with the same measuring cycle, but the FFT analyzer 4 needs to be operated with a different measuring cycle. Hence, the jitter analyzer calls for complicated control.
In addition, as mentioned above, the T-V converter 3 includes a D-A converter and a sample-hold circuit, whereas the FFT analyzer 4 includes a sample-hold circuit and an A-D converter. Thus, the prior art analyzer is, in its entirety, wasteful or uneconomical in construction.